Porous membranes composed of a hydrophobic high molecular weight polymers such as polyolefin exhibit excellent water resistance and chemical resistance. Further, these membranes are widely used for various applications, including the preparation of pure water for the electronics industry, manufacturing water purifiers for preparing potable water, and the preparation of sterilized water for medicines.
However, hydrophobic porous membranes are defective in that a liquid with a high surface tension, such as water is unable to permeate the membrane due to its low critical interfacial tension.
Therefore, to filter water or aqueous solutions through a hydrophobic porous membrane, previously it has been necessary to wet the fine pores of the membrane with a water-soluble organic solvent such as an alcohol, and then replace the solvent with water. However, this technique is defective in that, upon the removal of water, the membrane again acquires hydrophobic properties. Thus, when in filtering water or an aqueous solution through the membrane, the above-described processing with a hydrophilic organic solvent and subsequent replacement of the solvent by water must be repeated. Alternatively, the porous membrane must not be allowed to dry out. Consequently, the membrane must be kept in a state of being dipped in water. This involves the problem of inconvenience in preservation and transportation of the membrane and the possibility of change in quality of water during preservation. Thus, hydrophobic porous membranes possesing water-permeating properties without use of hydrophilic solvents or without preservation in water have been desired.
Techniques attempting to meet the above objective include conducting a corona discharge treatment and treating the filter with a strong oxidizing chemical agent. In addition, chemical surface-modifying processes such as irradiating a porous films surface with a hydrophilic monomer as described in JP-A-56-38333, plasma-treating a hydrophobic resin porous structure impregnated with a water-soluble high polymer or a surfactant as described in JP-A-56-157437, and depositing a nonionic ester of a hydrocarbon moiety and an organic mono-acid containing 8 to 30 carbon atoms (for example, mixture of sorbitan mono-esters of capric acid, lauric acid, myristic acid, palmitic acid and/or oleic acid, Span 20 (trade name)) onto a porous membrane as described in JP-A-59-501049, have been proposed. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, utilization of the corona discharge treatment results in a highly detrimental change in pore structure of the porous membrane. Further, permanent hydrophilic properties are not obtained. In addition, treating the filter with a strong oxidizing chemical agent has the defect that it seriously damages the substrate of porous membrane.
The radiation- or plasma-treating process is not practically applicable because it costs too much, though some slight improvement is observed.
Further, treatment with Span 20 results in a porous membrane which gives off such an offensive smell that it is generally not usable for use in water purifiers for obtaining potable water. However, the thus-treated membrane is rendered hydrophilic to a slight extent. Further, water obtained from a purification process utilizing the thus-treated membrane exhibits a foaming phenomenon.